1. Field of the Invention
The present invention relates to a crane having a multi-section extensible telescopic boom; and more particularly relates to a telescopic boom having angled corner sections with boom shoes slidable in said corner sections.
2. Description of the Prior Art
Conventionally the boom sections of telescopic booms for cranes or the like have been generally rectangular in transverse cross-section. Boom shoes, which are the contacting surfaces between the boom sections, are generally arranged in two sets, one set being disposed between adjacent vertical walls and the other set being disposed between adjacent horizontal walls. Because of the clearance required between the vertical boom walls and the associated side shoes, a horizontal side load applied to the tip of boom will cause the longitudinal axis of the tip section, as well as the axes of all other intermediate sections, to deflect out of alignment with the longitudinal axis of the base section in the direction of application of the side load. This side deflection or "kink" between the several boom sections increases with shoe wear and with multisection booms creates a large tip offset. The tip offset results in offset loading and increased side bending forces being applied to the boom resulting in detrimental side deflecting and stresses.
An additional problem occurs when the boom is swung about its vertical axis with a load suspended therefrom. The swinging action may cause the tip section to shift from one side to the other side of the longitudinal axis of the base section causing the suspended load to swing excessively making it difficult for the operator to accurately control the load and place the load in tight spots when swinging of the boom is required. With clearance between the side shoes and the vertical walls, as is conventional, the offset of the outer boom sections will cause additional side bending loading of the boom. Such side bending loading severely stresses welds or the like in the corners of the boom sections.
In addition to booms that are generally rectangular in cross-section, other boom shapes are known. For example, U.S. Pat. No. 3,708,937 which issued to Sterner on Jan. 9, 1973 discloses a trapezoidal boom section; Eiler et al Pat. No. 3,802,136 which issued on Apr. 9, 1974 discloses truncated triangular boom sections used in interconnected pairs; U.S. Pat. No. 3,985,234 which issued to Jouffray on Oct. 12, 1976 illustrates a boom section with the lower wall being V-shaped; and Lester et al Pat. No. 4,171,597 which issued on Oct. 23, 1979 discloses boom sections having external configurations which are generally octagonal. U.S. Pat. No. 4,257,201 which issued to Landolt et al on Mar. 24, 1981 discloses boom sections having slide pad assemblies that include cylindrical outer surfaces at their corners.